The ARIA in HTML specification already imposes this limitation on img-elements without an alt-attribute. Unless someone can come up with a good example of where this would be useful, I think it makes sense to disallow the textbox-role from being used on img-elements with an alt-attribute, too.

As an example of potential usability, @stevefaulkner mentioned a read-only textbox. If that were indeed a valid (and valuable) situation to cater to it might be worth requiring aria-readonly when setting the textbox role on an img.